Molecular Lemniscates from Organic-Metal Terpyridine-Based Self-Assembly and Host-Guest Recognition

Rhenium-Pyrazolyl-Based Figure-Eight- and Z-Shaped Metallocycles: Self-Assembly, Solid-State Structures, Dynamic Properties in Solution, and Competitive Ligand-Induced Supramolecular Transformations into Rhenium-Pyridyl/-Benzimidazolyl/-Phosphine-Based Metallocycles/Acyclic Complexes

Rhenium(I)tricarbonyl core-based heteroleptic "figure-eight"- and Z-shaped metallocycles (1a-4a) of the general formula fac-[{(CO)3Re(μ-L)Re(CO)3}2(dppz)2] were self-assembled from Re2(CO)10, H2-L (H2-L = 5,8-dihydroxy-1,4-naphthaquinone (H2-dhnq) for 1a; 1,4-dihydroxy-9,10-anthraquinone (H2-dhaq) for 2a; 6,11-dihydroxy-5,12-naphthacenedione (H2-dhnd) for 3a; 2,2'-bisbenzimidazole (H2-bbim) for 4a), and bis(4-((pyrazolyl)methyl)phenylmethane) (dppz) via one-pot coordination-driven synthetic approach. The molecular structures of 1a and 4a were unambiguously confirmed by single-crystal X-ray diffraction (SC-XRD) methods. The metallocycles in the DMSO solution exist as an acyclic dinuclear-DMSO adduct of the general formula fac-[{(CO)3Re(μ-L)Re(CO)3}(DMSO)2] (1b, L = dhnq; 2b, L = dhaq; 3b, L = dhnd; 4b, L = bbim) and dppz, which are in dynamic equilibrium. The dynamic behavior of the rhenium-pyrazolyl bond in the solution state was effectively utilized to transform metallocycles 1a-4a into pyridyl/benzimidazolyl/phosphine donor-based heteroleptic metallocycles and acyclic dinuclear complexes (4-13). These include tetranuclear rectangles fac-[{(CO)3Re(μ-L)Re(CO)3}2(4,4'-bpy)2] (4 and 11, L = dhaq for 4 and bbim for 11), dinuclear metallocycles fac-[{(CO)3Re(μ-L)Re(CO)3}(dpbim)] (5-7 and 12; L = dhnq for 5, dhaq for 6, dhnd for 7, and bbim for 12), and dinuclear acyclic complexes fac-[{(CO)3Re(μ-L)Re(CO)3}(PTA)2] (8-10 and 13; L = dhnq for 8, dhaq for 9, dhnd for 10, and bbim for 13). These transformations were achieved through component-induced supramolecular reactions while treating with competitive ligands 4,4'-bipyridine (4,4'-bpy), bis(4-((1H-benzoimidazole-1-yl)methyl)phenyl)methane (dpbim), and 1,3,5-triaza-7-phosphaadamantane (PTA). The reaction mixture in the solution was analyzed using NMR and electrospray ionization mass spectrometry (ESI-MS) analysis. Additionally, crystal structures of 4, 6, and 13, which were obtained in the mixture of the solutions, were determined, providing unequivocal evidence for the occurrence of supramolecular transformation within the system. The results reveal that the size of the chelating ligand and the pyrazolyl donor angle of the ditopic ligand play crucial roles in determining the resulting solid-state metallacyclic architecture in these synthetic combinations. The dynamic behavior of the rhenium-pyrazolyl bond in the metallocycles can be utilized to transform into other metallocycles and acyclic complexes using suitable competing ligands via ligand-induced supramolecular transformations.

Solvent and Counteranion Assisted Dynamic Self-Assembly of Molecular Triangles and Tetrahedral Cages

Self-assembly of naked Pd^II ions separately with newly designed bis(3-pyridyl)benzothiadiazole (L1) and bis(3-pyridyl)thiazolo[5,4-d]thiazole (L2) donors separately, under varying experimental conditions, yielded Pd₄L₈ (L= L1 or L2) tetrahedral cages and their homologous Pd₃L₆ (L= L1 or L2) double-walled triangular macrocycles. The resulting assemblies exhibited solvent, temperature, and counteranion induced dynamic equilibrium. Treatment of L1 with Pd(BF₄)₂ in acetonitrile (ACN) resulted in selective formation of a tetrahedral cage [Pd₄(L1)₈](BF₄)₈ (1a), which is in dynamic equilibrium with its homologue triangle [Pd₃(L1)₆](BF₄)₆ (2a) in dimethyl sulfoxide (DMSO). On the other hand, similar self-assembly using L2 instead of L1 yielded an equilibrium mixture of tetrahedral cage [Pd₄(L2)₈](BF₄)₈ (3a) and triangle [Pd₃(L2)₆](BF₄)₆ (4a) forms in both ACN and DMSO. The assembles were characterized by multinuclear NMR and ESI-MS while the structure of the tetrahedral cage (1a) was determined by single crystal X-ray diffraction. Existence of a dynamic equilibrium between the assemblies in solution has been investigated via variable temperature ¹H NMR. The equilibrium constant K = ([Pd₄L₈]³/[Pd₃L₆]⁴) was calculated at each experimental temperature and fitted with the Van't Hoff equation to determine the standard enthalpy (ΔH°) and entropy (ΔS°) associated with the interconversion of the double-walled triangle to tetrahedral cage. The thermodynamic feasibility of structural interconversion was analyzed from the change in ΔG°, which suggests favorable conversion of Pd₃L₆ triangle to Pd₄L₈ cage at elevated temperature for L1 in DMSO and L2 in ACN. Interestingly, similar self-assembly reactions of L1 and L2 with Pd(NO₃)₂ instead of Pd(BF₄)₂ resulted in selective formation of a tetrahedral cage [Pd₄(L1)₈](NO₃)₈ (1b) and double-walled triangle [Pd₃(L2)₆](NO₃)₆ (4b), respectively.

Self-assembled metallasupramolecular cages towards light harvesting systems for oxidative cyclization

Designing artificial light harvesting systems with the ability to utilize the output energy for fruitful application in aqueous medium is an intriguing topic for the development of clean and sustainable energy. We report here facile synthesis of three prismatic molecular cages as imminent supramolecular optoelectronic materials via two-component coordination-driven self-assembly of a new tetra-imidazole donor (L) in combination with 180°/120° di-platinum(ii) acceptors. Self-assembly of 180° trans-Pt(ii) acceptors A1 and A2 with L leads to the formation of cages Pt4 L 2(1a) and Pt8 L 2(2a) respectively, while 120°-Pt(ii) acceptor A3 with L gives the Pt8 L 2(3a) metallacage. PF6 - analogues (1b, 2b and 3b) of the metallacages possess a high molar extinction coefficient and large Stokes shift. 1b-3b are weakly emissive in dilute solution but showed aggregation induced emission (AIE) in a water/MeCN mixture as well as in the solid state. AIE active 2b and 3b in aqueous (90% water/MeCN mixture) medium act as donors for fabricating artificial light harvesting systems via Förster resonance energy transfer (FRET) with organic dye rhodamine-B (RhB) with high energy efficiency and good antenna effect. The metallacages 2b and 3b represent an interesting platform to fabricate new generation supramolecular aqueous light harvesting systems with high antenna effect. Finally, the harvested energy of the LHSs (2b + RhB) and (3b + RhB) was utilized successfully for efficient visible light induced photo-oxidative cross coupling cyclization of N,N-dimethylaniline (4) with a series of N-alkyl/aryl maleimides (5) in aqueous acetonitrile with dramatic enhancement in yields compared to the reactions with RhB or cages alone.